Open source software is essentially free software that allows users to download and modify the source code. This allows millions of programmers around the world to create new innovative plug-ins that are added to Word Press and other open source systems from photo galleries, newsletters, social media integration for a fraction of the cost of proprietary systems.
One of the fundamental differences between open source software and proprietary software is that the source code of open source software must be made freely available with the software. Anyone should be able to download the source code, view it, and alter it as they see fit. With proprietary software, you generally cannot view or edit the source code.
Organizations that engage with open source software can protect themselves to some extent from vendor lock-in, or the possible obsolescence of the software they use.